Field of the Invention
The present invention relates to a polycrystalline diamond film comprising diamond crystallites and a method of growth of the polycrystalline diamond film.
Description of Related Art
Diamond is the hardest material known, having a Mohs Hardness of 10, which makes diamond most useful for applications of cutting, machining, drilling, milling, etc. Diamond is also the most thermally conductive material known, having a thermal conductivity up to 2000 to 2200 watts per meter per Kelvin, which makes it highly desirable for applications in thermal management under demanding conditions. Diamond also has an extremely low coefficient of friction, which makes it a versatile material for uses as brakes. Diamond is also an excellent optical material for transmitting microwave, infrared, visible, and other ultraviolet electromagnetic waves. Diamond has a high stability when used as detector for high fluence nuclear radiation. In addition, diamond is also highly inert in chemical environments that involve strong acid, strong base, strong oxidizing agent, or strong reducing agent, even at elevated temperatures or at cryogenic conditions. Furthermore, diamond is a high refractive index material, which leads to its popularity and premium value in jewelry industries. For more information, please refer to following references, (1) “Properties, Growth and Applications of Diamond”, Edited by M. H. Nazare and A. J. Neves, 2001, published by The Institute of Electrical Engineers; (2) “Diamond Films Handbook”, edited by Jes Asmussen and D. K. Reinhard, 2002, published by Marcel Dekker; and (3) “Diamond Films, Chemical Vapor Deposition for Oriented and Heteroepitaxial Growth”, Edited by Koji Kobashi, 2005, published by Elsevier.
Though diamond is one of the most versatile and most premium materials, its availability is very limited in nature. Diamond mined from the earth is typically of single crystal with geometrical dimensions that are limited in size, most of the time, too small for industrial uses that require large dimensions. Many times, diamond formed in nature contains impurities and crystal defects. The diamond crystal that is relatively large in crystal size, relatively pure in chemical contents, and relatively perfect without crystal defects is very expensive, often times, priceless.
Synthetic diamond is known to be produced industrially in chemical reactors under extremely High Temperatures and extremely High Pressures, known as the HTHP process. Due to such harsh synthetic conditions, reactor sizes are limited, as are the dimensions of the diamond grown from the HTHP process, not to mention its associated high costs in process, equipment, and safety. Often times, the HTHP process produces diamond that has a yellow tint due to the incorporation of catalytic impurities into diamond lattices. In addition, the HTHP process is not able to produce diamond wafers of a large diameter.
Industrially, single crystal diamond can also be grown in reactors in a process called chemical vapor deposition (CVD), where suitable growth conditions can be achieved by microwave-enhanced plasma, tungsten hot-filament, DC-Jet plasma, laser-induced plasma, acetylene-torch, etc. It is known in art that CVD growth processes can also successfully grow polycrystalline diamond thin films on different substrates and/or free standing diamond thick films, though very challenging to obtain low stress films or non-cracked diamond of significant size. However, the CVD growth process can produce diamond substrates that can be significantly greater in size than the diameter of single crystal diamond from nature or grown using the HTHP process. Nevertheless, the growth rate of diamond in CVD process, or any other diamond growth process, is generally slow, e.g., in a range from a growth rate of less than 1 micron/hr to a growth rate of a few microns/hr, though there are reports of being able to grow single crystal at a higher growth rate, but with an increased number of defects.
For economic reasons, it is desirable to grow diamond film at a high growth rate with a large diameter, thus resulting in reduced production cost per unit volume of diamond. Higher growth temperature and higher methane concentration can drive the growth rate of diamond film. However, growing large diameter diamond films at high growth rates has challenges. The faster the diamond film grows, the more undesirable Sp2 carbon atoms are incorporated into the diamond film along with the desirable Sp3 carbon atoms due to a lack of time for hydrogen free radicals to etch away the undesirable Sp2 carbon atoms from the diamond film, which results in an increased stress in the diamond film, and, often times, an undesirable quality of the diamond film. The faster the diamond grows, at the same time, the more diamond crystallites can be misaligned on the diamond film, which can also result in a higher level of stress in the diamond film. In addition, the larger the diameter of the substrate on which diamond grows, the more the stress is accumulated in the diamond film, which can result in undesirable premature delamination and shattering of the diamond film. Therefore, the challenge is to successfully grow diamond film fast and large with desirable features, attributes and properties for various applications.
Even though diamond is an extremely hard material, the hardness of a single crystal diamond varies with its crystal orientation. A surface perpendicular to the [111] crystallographic direction of a pure diamond is the hardest. A surface perpendicular to the [111] is 100 times harder than a surface perpendicular to the [100] crystallographic direction of a pure diamond. Therefore, the [111] direction of the diamond crystal is the most durable and most desirable for mechanical applications with respect to wear resistance, while the [100] direction is softest and most desirable for its level of easiness being machined/fabricated to various tool shapes.
CVD polycrystalline diamond for mechanical uses can be grown into a free-standing wafer with a thickness from a few hundred microns to as thick as a couple of thousand microns, sometimes as thick as 3,000 microns or more. An as-grown free standing diamond wafer can be used for mechanical applications without lapping or polishing. The surface of as-grown free standing diamond wafer can be optionally lapped, and can be optionally further polished, to a desired surface roughness and flatness. Then, an as-grown, surface-lapped and/or surface-polished free standing diamond wafer can be laser-cut, or electrical-discharge-milled (EDM), if the diamond film is doped for electric conductivity, into different geometries for mechanical applications such as dressing, cutting, milling, drilling, grinding, lathing, etc. The surface that is generated by laser-cutting or EDM is used as a working surface for mechanical purposes. Many times, such working surface needs to be machined or fabricated at a certain angle. Before its use, the newly-generated laser-cut or EDM surface is often finely grounded to precise and desirable geometric shape and finish. Therefore, it is very desirable to have a level of ease in machining or fabricating the needed angle of the working surface. For a piece of single crystal diamond, there are choices to machine or fabricate on the surface along a direction of the [100] of the diamond crystal lattice. For polycrystalline free-standing CVD diamond, there is no such choice unless all diamond crystallites are preferentially oriented into to certain directions. Accordingly, it would be desirable to have a [100] oriented surface on which a needed angle can be relatively easily machined or fabricated. Additionally, it would also be desirable to form a polycrystalline free-standing diamond tool having a working surface that is preferentially-oriented to the [111] direction so that this diamond tool is harder, which can work better, have a lower cost of ownership; and have less frequency of tool change, which is more efficient and requires lower investments in capital equipment, etc.
In summary, it would be desirable to successfully grow a polycrystalline diamond film at a high growth rate on a large diameter substrate to a desirable thickness with a level of benign stress that prevents premature delamination. It would also be desirable that such polycrystalline diamond film is highly-oriented to the [110]-orientation along growth direction, and also has preferential orientations (the [111] and [100] directions) on cross-section of the diamond film, at certain angles from the growth surface, in a way that such diamond film is easy to fabricate into different tool geometries ([100] direction preferred), as well as has a desired level of hardness (the [111] direction preferred) for various applications such as, but not limited to, mechanical, thermal management, optics, detectors, wear-resistance, chemical inertness, acoustic, electromagnetic wave management, etc. It would also be desirable that such polycrystalline diamond film behave like a polycrystalline diamond film of small grain size in disrupting the [111] crystal cleavage planes for attaining desirable mechanical durability, but while allowing phonons to transport to attain a desired level of thermal conductivity.